It's Elo not ELO. Elo is not an acronym.

https://en.wikipedia.org/wiki/Elo_rating_system

That’s a fun fact!

From what I've seen in the comments, it's definitely ELO, if not through ubiquity alone. Happens to the best of 'em!

Elo is nicer as it gives a nod to the inventor, no?

Some say the greatest compliment a mathematician can be paid is when a concept that bears their name is so ubiquitous that we no longer capitalise it. (E.g. Abel with the abelian group).